I. FIELD OF THE INVENTION
This invention relates to the field of signal processing and in particular to the elimination of jitter from phase encoded signals in a communications receive channel.
II. BACKGROUND ART
Communication of data information over the voice network between a sending and receiving station is typically accomplished with a modem (modulator/demodulator). A modem transforms digital information by modulating it according to one of several well-known modulating schemes. This modulated data is transmitted to the receiving modem, where it is demodulated to produce the original information. In order to properly demodulate and decode transmitted information, the clock phase and carrier phase of the transmission must be known to the demodulator. Typically, timing information is taken from the transmitted signal itself by means of various techniques.
The frequency of the line voltage varies from country to country but is generally in the range of 50 to 60 Hz. Due to the use of this power source in telephone equipment, jitter can be introduced to communication transmissions. Such jitter can cause distortion in an information signal and can hinder decision making of decoding circuitry. Jitter may interfere with the definition of baud windows as it introduces phase shifts, etc., into the information signal. In addition, carrier phase jitter may introduce excess noise in the signal path, particularly with QAM and DPSK transmission, effectively preventing reception of such signals over certain telephone lines.
In order to reduce the effects of such jitter, it must be detected and eliminated. Because the line voltage frequency varies from country to country, any such jitter tracking circuitry must operate over a band of frequencies.
Therefore, it is an object of the present invention to provide a jitter tracker which detects and eliminates jitter components at harmonics of the power line voltage frequency associated with communication systems.
It is a further object of the present invention to provide a jitter tracker which operates over a band of frequencies, particularly in the range of 50 to 60 Hz.
It is yet another object of the present invention to provide a jitter tracker with phase correction capabilites.